Two-channel rotary wave guide joint



Filed Augg. 7, 1953 H. P. RAM/ABE TWO-CHANNEL ROTARY WAVE GUID JOINT 2 Sheets-Sheet l INVENTOR.

May m, 195@ H. P. mmm

Two-CHANNEL ROTARY WAVE GUIDE JOINT 2 Sheecs-Sheet 2 Filed Aug. 7. 1.953

INVENTOR.

United (Granted 3d', "U. Code (i952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without payment to me of any royalty thereon. This invention relates to rotating joints for microwave transmission circuits and particularly to a two-channel f rotating joint in which the frequency bands of the two channels are the same or else very close together in the frequency spectrum. Such joints are useful, for example, in cases where it is desired to convey energy between stationary radio equipment and a pair of antennas mounted on a rotating structure.

Double rotary joints of both the coaxial transmission line type and. the wave guide type are known. In the coaxial line ltype there are three concentric conductors the middle one of which acts as the outer conductor of the inner channel and the inner conductor of the outer channel. T he principal disadvantage of this type, in addition to its size, is the necessity of contact for three conductors between the stationary and rotary parts of the joint. The wave guide type of double rotary joint comprises a pair of concentric circular wave guides with the energy of one channel passing through the space of the inner cylinder and the energy of the other channel passing through the space between the inner and outer cylinders, circularly symmetrical E or H modes being used. With E modes the Contact problem is reduced to two conductors while with the H modes the contact problem is eliminated completely since no longitudinal current is established by these modes. Combinations of the coaxial line type and the coaxial wave guide type are also known. However, all of these types have the one common feature that the two channels are guided through perfectly separated spaces which require at least two concentric cylindrical conductors. In addition, there is also known a multiple rotary wave guide joint comprising a single circular wave guide in which the rotating joint is located and through which the energies of a number of channels are transmitted in the same circularly symmetrical mode. However, in this case, it is necessary that suflicient separation exist between the frequency bands of the various channels to permit their separation by suitable lters on either side of the rotating joint.

it is the object o-f this invention to provide a two-channel rotary wave guide joint in which the two channels are transmitted through a single circular wave guide and in which the frequency bands of the two channels may be the same or very close together.

The rotary joint constituting the invention may be classified as a multiple mode rotary joint. Its basic principle is that, if the diameter of a circular wave guide is large enough to allow transmission of more than one circularly symmetrical mode, and if it is possible `to excite these modes individually, there will be no crosstalk between the channels during simultaneous transmission. Briey, the rotary joint comprises two pairs of rectangular wave guide terminals, a circular wave guide which transmits both channels, and coupling elements between the rectangular rates Patent wave guide terminals and the circular wave guide which accomplish transitions between the rectangular H10 mode and the circular H01 and E01 modes. While the transition to the E01 mode is performed by a conventional method it was necessary to devise coupling means for achieving the transition between the rectangular H10 mode and the circular H01 mode, and, accordingly, such means are a further object of this invention. Briefly, this coupling means comprises at least three and preferably four resonant .slots equally spaced around the cylindrical wall of the circular wave guide and fed in the same amplitude and phase by a rectangular wave guide surrounding the circular guide. The electric fields produced at the slots contribute to the transverse circular electric field of the circular H01 mode. By the use of this coupling means and by a judicious choice of the circular wave guide diameter the generation of undesired modes is prevented, aS will be explained more fully later.

A more detailed description of the invention will be given in connection with the specific embodiments thereof shown in the accompanying drawings, in which Fig. l illustrates various modes in circular wave guides;

Fig. 2 shows a two-channel rotary joint in accordance with the invention;

Figs. 3, 4, 5 and a-e show various coupling means usable with the apparatus of Fig. 2, for electing a.y transition between the rectangular H10 mode and the circular H01 mode, and

Fig. 7 is a vector diagram illustrating the operation of the coupling means of Figs. 6ae.

Figure l shows the first eight circular modes arranged in the order of their minimum diameters normalized with respect to the free space wavelength. From this figure it is seen that the circular symmetrical modes having the smallest and next smallest minimum diameters are the E01 and H01, respectively. These two modes are therefore indicated for a two-channel rotary joint. Coupling means usually excite a number of modes, however, only those modes having minimum diameters equal to or less than the diameter of the wave guide will be transmitted. As a result, if the diameter of the circular wave guide in this case is made greater than 1220A, the minimum diameter for the H01 mode, and less than 1338A, the minimum diameter for the H01 mode, the H31 and higher modes are not transmitted. The coupling devices must be designed to prevent excitation of the undesired modes having minimum diameters less than the wave guide diameter, namely, the H11, H21 and E11 modes.

A rotary joint in accordance with the invention is shown in Fig. 2. The circular wave guide 1 has a diameter not less than 1220A, the minimum diameter of the H01 circu lar mode. Terminal rectangular wave guides 2 and 3, operating in the dominant rectangular mode H10, are coupled to the circular guide l by means of coupling rods 4. and 5 extending through holes in the centers of circular guide end plates e and 7. This is a conventional method of coupling between the rectangular H10 mode and the circular E01 mode. None of the undesired modes capable of transmission in guide 1 are excited byv this coupling arrangement.

The transition between the H10 mode in terminal rectangular wave guides 3 and 9 and the circular H01 mode of wave guide l is accomplished by four equally spaced slots in each end of the circular guide, two of which Vare shown in Fig. 2, and means, interposed between the terminal guides 8 and 9 and the slots, for feeding the slots in each group in the same amplitude and phase. When so fed the electric iields produced by the slots contribute to the circular transverse iield of the H01 circular mode' and will not excite any other mode having a smaller minimum diameter than the H01 mode. This coupling arrangement is of course bi-drectional and will receive energy from the H01 mode as well as contribute energy thereto. The slots are proportioned to be resonant at the design frequency. The spacing between the center of each slot and the end of the circular wave guide should be one-quarter wave length while the spacing between the centers of transmitting and receiving slots should be a multiple of one-half wave length.

The H01 mode may also be excited by one, two or three slots,f however, one slot would also excite undesired modes H11, H21 and E11, and two slots would excite the undesired mode H21. Therefore, a minimum of three slots is required to avoid excitation of undesired modes ylower than the H01 mode. Three slots, however, will excite the H31 mode which has a minimum diamter only slightly greater than that of the H01 mode. In order to avoid the necessity for designing the circular wave guide 1 within the narrow diameter range of 1.220% to 1338A, four slots are used to prevent excitation of the undesired H31 mode. The next higher mode excited by four slots is the H41 which is not transmitted since its minimum diameter exceeds the wave guide diameter.

A number of methods of feeding the slots in wave guide 1 are possible. One arrangement which may be used in Fig. 2 is shown in Fig. 3. The terminal wave guide 8 is continued around the circular wave guide and contains slots --13 in its inner wall which may be common with the wall of circular guide 1. In order to have equal amplitudes and phases for the electrical fields generated by the slots they must be separated by a mulitiple of one guide wavelength. The Wave guide is terminated by a short circuit` at `a distance of onehalf guide wave length beyondV the last slot so that maximum current density exists around the slots. This arrangement has a limited useful band width since a deviation of the frequency unbalances the amplitude and phase of the four slots and consequently sets up undesired modes.

A coupling arrangement having a Wider band width than that of Fig. 3 is shown in Figs. 2 and 4. A rectangular wave guide 14, having one-half the height of terminal wave guide 8, surrounds circular guide 1 and contains `slots 10-13 in its inner wall which may be common with the wall of the circular guide. Impedance matching metallic prism serves to reduce reections at the T-junction. The two waves travelling around in opposite directions establish a node of the electrical field at a point equidistant from the T-junction so that this point acts like a short circuit. If the slots are separated by one guide wave length the amplitudes and phases of the electric fields produced by all four slots will be the same as the design frequency. At other frequencies balance is maintained between slots 10 and 13 and between slots 11 and 12; but an unbalance depending lupon the frequency deviation does occur between slots 10 and 11 and between slots 13 and 12. This unbalance however is much less than the imbalance for the same frequency deviation in the arrangement of Fig. 3.

Y VA coupling arrangement in which lthe balance in ampli- Cil tude and phase of the electrical lields produced by the four slots is independent of frequency is shown in Fig. 5. In this arrangement a rectangular wave guide 16 completely surrounds circular wave guide 1, and its `inner wall, which may be common with the wall of guide 1, contains the four slots lll-13. Wave guide 16 is surrounded by a second rectangular wave guide 17, the inner wall of which may be common with the outer wall of guide 16. Guide 17, which has one-half the height of terminal guide 8, forms a T-junction with the terminal guide. Energy transfer between guides 17 and 16 is effected through slots 18 and 19 which are located midway between slots 10 and 11 and 12 and 13, respectively, and which are equidistant from the T-junction. Due to ,the symmetry of this arrangement deviations in frequency affect all slots alike so that their balance is independent of frequency.

Figures 6ft-e show an arrangement by which the result obtained in Fig. 5 can be obtained with a single surrounding wave guide. ln this embodiment, square wave guide 20 surrounds the wave guide 1 and includes slots 10-13 in its inner wall which may be common with the wall of wave guide 1. The square wave guide is v coupled to the terminal rectangular wave guide by means of a transition section 21. Energy transfer between the `square wave guide and the transition section is through a resonant horizontal slot 22 which passes the vertically polarized H10 mode of the rectangular wave guide. From the T-junction two waves equal in amplitude and phase travel in both directions around the square Wave guide. Since the H10 Waves are vertically polarized they will not excite the vertical slots 10-13 nor will they pass the vertically slotted barrier 23 located at a point diametrically opposite the T-junction. Consequently, the barrier 23 acts as a short circuit from which the Waves are reflected so that identical standing waves are established in each half of the wave guide 20. The square wave guide contains corner slots 25 and 26 located at diametrically opposite points equidistant between the T-junction and the barrier 23. lf the square wave guide has an electrical length equal to four guide wave lengths, identical current maxima occur at these slots, however, the equality of these two currents in amplitude and phase is maintained at all frequencies.

The vertically polarized, square H10 mode that travels in both directions from the T-junction may be resolved into two diagonally polarized modes at right angles to each other, as represented in Fig. 7. In this figure vector 27 represents the vertically polarized H10 mode. This vector may be resolved into vectors 28 and 29, representing the diagonally polarized rnodes, each of which may in turn be resolved into vertically and horizontally polarized square H10 and H01 modes as represented byv vectors 30, 31, 32 and 33. The sum of vectors 30-33 equals vector 27.

The eifect of each of slots 25 and 26 is to stop the current flow of the diagonal mode pointing into the slotted corner, causing this mode to be reiiected as from an open end. Assuming that the electric lield of the diagonal mode represented by vector 28 is along the diagonal of the'wave guide 20 passing through slot 25, this mode will be reflected in a clockwise direction from slot 25. The diagonal mode represented by vector 29, however, is not affected by slot 25 and continues in a counterclockwise direction. Considering the H01 components of reflected Inode 2S and unreflected mode 29, slot 25 acts as a source `of H01 energy traveling in both directions around the wave guide 20 and in this respect is analogous to slot 19 in Fig.'5. This horizontally polarized energy is passed by vertically slotted barrier 23, but is rejected by H-plane slot 22, and excites the slots 10-13 in proper phase t0 contribute to the circular electric field `of the circular H01 mode in wave guide 1. In a similar manner slot 26 acts as a source of square H01 energy travelling in both directions therefrom and, in this respect, is analogous to slot 18 in Fig. 5. The operation is therefore similar to Fig. 5 and the balance with respect to both phase and amplitude of the electric elds produced by `slots 10-13 is independent of frequency.

It is necessary to place slot 26 in the lower corner of wave guide 20, rather than in the upper corner symmetrically with slot 25, in order to achieve an additive phase condition for the two H01 waves arriving at each of slots lil-13. The two waves travelling in opposite directions from the T-junction in Figs. 6a-e have the same phase, whereas the two waves leaving the T-junction in Fig. 5 have opposite phases. Therefore, in order to create the same conditions at slots 25 and 26 as exist at slots 19 and 18 of Fig. 5, a phase reversal at one of the slots is required. This is accomplished by placing slot 26 4in the lower rather than the upper corner of wave guide 20.

Radiation from slots 25 and 26 may be prevented by a quarter-wave shorted stub as shown in Fig. 6e.

Any of the above described coupling means for effecting a transition between the rectangular H10 mode and the circular H01 mode may be used in the rotating wave guide joint of Fig. 2.

The mechanical rotating joint located in the circular wave guide l and generally indicated at 34 may be of any known type providing low impedance to the longitudinal currents of the E01 mode. A conventional rotating choke joint is shown in Fig. 2. The L-shaped slot has a length equal to one-half wave length of the E01 mode so that its impedance at the inner surface of guide 1 is theoretically zero. The rotating bearing is placed at the midpoint of the half-wave slot, where the current is theoretically Zero, so that good Contact is not required. Since the circular H01 mode produces no longitudinal current there is no contact problem with respect to this mode.

I claim:

l. A two-channel rotary wave guide joint assembly comprising a circular wave guide having a rotary joint intermediate its ends and means for coupling each of said channels to said circular wave guide on opposite sides of said rotary joint, the coupling means for one channel providing a transition between the energy in the channel and the circular E01 mode and the coupling means for the other channel providing a transition between the energy in the channel and the circular H01 mode.

2. Apparatus as claimed in claim l in which the coupling means to the H01 mode, on each side of said rotary joint, comprises at least three longitudinal slots spaced equidistantly around the wall of said circular wave guide and means for feeding said slots so that the voltages thereacross are equal in amplitude and phase.

3. A two-channel rotary wave guide joint assembly comprising a circular wave guide having closed ends and a rotary joint intermediate its ends, the diameter of said circular guide lying between the minimum diameter for the H01 circular mode and the minimum diameter for the H11 circular mode, and means for coupling each of said channels to said circular wave guide on opposite sides of said rotary joint, the coupling means for one channel providing a transition between the H14 rectangular mode and the E01 circular mode, and the coupling means for the other channel providing a transition between the rectangular H10 mode and the circular H01 mode and comprising, on each side of said rotary joint, four resonant longitudinal slots spaced equidistantly around the wall of said circular guide and located equidistantly from the end thereof, and means for feeding said slots so that the voltages thereacross are equal in amplitude and phase.

4. Apparatus as claimed in claim 3 in which the centers of said slots are spaced from the adjacent end of said circular guide by one-quarter guide wave length, and the centers of the slots on opposite sides of said rotary joint are spaced by a multiple of one-half guide wave length.

5. Apparatus as claimed in claim 3 in which said feeding means comprises, on each side of said rotary joint, a rectangular wave guide partly surrounding said circular wave guide and containing said four slots in its inner wall, said slots being spaced circumferentially by one guide wave length, one end of said rectangular wave guide serving as a terminal and the other end being terminated in a short circuit at a distance of one-half guide wave length beyond the slot farthest removed along the wave guide from said terminal.

6. Apparatus as claimed in claim 3 in which said feeding means comprises, on each side of said rotary joint, a first rectangular wave guide completely surrounding said circular wave guide and containing said four slots in its inner wall, a second rectangular wave guide completely surrounding said rst wave guide and having an inner wall common with the outer wall of said first guide, a

pair of diametrically opposite slots in said common wall midway between opposite pairs of said four slots, and a terminal rectangular wave guide making a T-junction with said second guide at a point midway between said pair of slots.

7. Apparatus as claimed in claim 3 in which said feeding means comprises, on each side of said rotary joint, a square waveguide completely `surrounding said circular wave guide and containing said four slots in its inner wall, a rectangular terminal wave guide, a T-junction between said terminal wave guide and said square wave guide at a point midway between two of said four slots, said T-junction comprising a rectangular-to-square transition wave guide coupled to said square wave guide through an H -plane resonant slot in the outer wall of said square wave guide, a transverse wall having a resonant slot therein perpendicular to the H-plane of the square H10 mode located at a point diametrically opposite said T-junction, and a pair of diametrically opposite corner slots located in said square wave guide at points midway between said T-junction and said Wall, said corner slots being at oppcsite ends of a vertical wall of said square wave guide.

8. Apparatus as claimed in claim 3 in which said means for providing a transition from the rectangular H10 mode to the circular H01 mode comprises, on each side of said rotary joint, a coupling rod extending axially through a hole in the center of the circular wave guide end closure.

9. A wave guide coupling means for eifecting a transition between the H10 mode of a rectangular wave guide and the H01 mode of a circular wave guide without eX- citing any circular modes having minimum diameters less than that of the H01 mode, said coupling means comprising four longitudinal resonant slots at the same axial position in said circular guide and equally spaced around the wall of said circular guide, a terminal rectangular wave guide partly surrounding said circular wave guide and containing said four slots in its inner wall, said slots being spaced circumferentially by one guide wave length, and a short circuit termination of said terminal wave guide at a distance of one-half guide wave length beyond the slot farthest removed along the wave guide from its terminal end.

l0. A wave guide coupling means for etecting a transition between the H10 mode of a rectangular wave guide and the H01 mode of a circular wave guide without exciting any circular rnodes having minimum diameters less than that of the H01 mode,.said coupling means comprising four longitudinal resonant slots at the same axial position in said circular guide and equally spaced around the wall of said circular guide, a iirst rectangular Wave guide completely surrounding said circular Wave guide and containing said four slots in its inner wall, a second rectangular wave guide completely surrounding said first wave guide and having an inner wall common with the outer wall of said first guide, a pair of diametrically opposite slots in said common wall midway between opposite pairs of said four slots, and a terminal rectangular wave guide making a T-junction with said second guide at a point midway between said pair of slots.

1l. A wave guide coupling means for effecting a transition between the H10 mode of a rectangular wave guide and the H01 mode of a circular wave guide without exciting any circular modes having minimum diameters less than that of the H01 mode, said coupling means comprising four longitudinal resonant slots at the same axial position in said circular guide and equally spaced around the wall et said circular guide, a square wave guide cornpletely surrounding said circular wave guide and containing said four slots in its inner wall, a rectangular terminal wave guide, a T-junction between said terminal wave guide and said square wave guide at a point midway between two of said four slots, said T-junction comprising a rectangular-to-square transition wave guide coupled to said square wave guide through an H-plane resonant slot in the outer wall of said square wave guide, a transverse wall having a resonant slot therein perpendicular to the H-plane of the square H10 mode located at a point diametrically opposite said T-junction, and a pair of diametrically opposite corner slots located in said square Wave guide at points midway between said T-junetion and said Wall, said corner slots being at-opposite ends of a vertical Wall of said square wave guide.

References Cited in the le of this patent UNITED STATES PATENTS 2,088,749 King Aug. 3, 1937 8 Bradley May 24, Roberts June 27, Mieher Sept. 26, Ortusi Dec. 19, Lanciani Apr. 20, Farr July 12, Preston Sept. 27, Miller May 29, 

